Chromium plating method

ABSTRACT

A chromium plating method wherein an article to be plated is immersed in an acidic chromium electroplating bath and electrolysis is carried out using a positive electrode that has an iridium oxide-containing film at least on the surface. The acidic chromium electroplating bath contains a trivalent chromium compound and a hexavalent chromium compound at a ratio such that the total chromium concentration of trivalent chromium and hexavalent chromium is 60-140 g/L, the hexavalent chromium concentration is 5-40 g/L, and the ratio of the hexavalent chromium concentration is 5-35% by mass of the total chromium concentration, while containing 50-400 g/L of organic carboxylate ions and having a lead ion concentration of not more than 2 mg/L. By the method, a chromium plating film that is good and stable over a long time period can be obtained. In addition, the plating bath can be controlled extremely easily in the chromium plating method.

TECHNICAL FIELD

The present invention relates to a chromium plating method that employs a plating bath containing a trivalent chromium compound and a hexavalent chromium compound mixed together.

BACKGROUND ART

Among well-known conventional chromium plating baths are the one which contains chromic acid (which is a hexavalent chromium compound) as the major component and the one which contains a trivalent chromium compound. The bath containing chromic acid as the major component is widely used as one of them. Beside, the bath containing a trivalent chromium compound is beginning to be used in the environmental point of view. However, the conventional plating bath containing a trivalent chromium compound causes troubles when it is contaminated with hexavalent chromium ions (Cr⁶⁺).

On the other hand, there is also known a chromium plating bath which contains a trivalent chromium compound and a hexavalent chromium compound in combination. (This is referred to as a combined chromium plating bath hereinafter.) See Patent Documents 1 to 4 and Non-Patent Documents 1 to 8 given below.

However, the plating method with the combined chromium plating bath is almost not industrially used at present. The reason for this is that chromium plating with the conventional combined chromium plating bath proceeds smoothly in its early stage but is liable to cause troubles after a comparatively short period of time, thereby preventing stable plating operation.

PRIOR-ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-B S46-40761 -   Patent Document 2: JP-A S52-125427 -   Patent Document 3: JP-A S59-185794 -   Patent Document 4: JP-A S59-223143 -   Patent Document 5: JP-A H03-260097 -   Patent Document 6: Japanese Patent No. 3188361 -   Patent Document 7: Japanese Patent No. 3810043

Non-Patent Documents

-   Non-Patent Document 1: “Formation of Electro-deposits of Bright     Chromium from Chromic Acid Bath, containing Saturated Dicarboxylic     Acids”, by Seiichiro Eguchi, Kinzoku Hyomen Gijutu (Metal Surface     Technology), vol. 19, No. 11, p. 451-456, 1968 -   Non-Patent Document 2: “Preparation of Amorphous Cr and Amorphous Cr     Binary Alloys”, by Hisashi Furuya, Yoshinari Misaki, and Yoshimi     Tanabe, Kinzoku Hyomen Gijutu (Metal Surface Technology), vol. 32,     No. 12, p. 631-636, 1981 -   Non-Patent Document 3: “Bath Composition and Electolytic Condition     for Decorative Chromium Plating from Oxalic Acid Baths”, by     Seiichiro Eguchi and Tooru Yoshida, -   Non-Patent Document 4: “Bath Voltage and Covering Power of Chromium     Plating from Oxalic Acid Baths” by Seiichiro Eguchi, Tsutomu     Morikawa, and Masayuki Yokoi, Kinzoku Hyomen Gijutu (Metal Surface     Technology), vol. 35, No. 2, p. 104-108, 1984 -   Non-Patent Document 5: “Hardness of Chromium Deposits from Oxalic     Acid Baths” by Tsutomu Morikawa and Seiichiro Eguchi, Kinzoku Hyomen     Gijutu (Metal Surface Technology), vol. 37, No. 7, p. 341-345, 1986 -   Non-Patent Document 6: “Preparation of Cr—C Alloy Plating from     Cr(III) Sulfate-Carboxylate Baths”, by Tsutomu Morikawa, Masayuki     Yokoi, Seiichiro Eguchi, and Yukio Fukumoto, Hyomen Gijutu (Surface     Technology), vol. 42, No. 1, p. 95-99, 1991 -   Non-Patent Document 7: “Amorphous Cr—C Alloy Plating from Cr(III)     Sulfate-Amorphous Oxalate Bath”, by Tsutomu Morikawa, Masayuki     Yokoi, Seiichiro Eguchi, and Yukio Fukumoto, Hyomen Gijutu (Surface     Technology), vol. 42, No. 1, p. 100-104, 1991 -   Non-Patent Document 8: “Decorative Trivalent Chromium Plating”, by     Kazuo Watanabe, Hyomen Gijutu (Surface Technology), vol. 56, No.     6, p. 320-324, 2005

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention improves the foregoing circumstances. An object of the present invention is to provide an industrially advantageous chromium plating method which permits continued operation with the above-mentioned combined chromium plating bath for a long period of time.

Means for Solving the Problems

The present inventors have earnestly studied in order to attain the above object. As a result, it has been found that a good chromium plated film can be favorably obtained by using an acidic electroplating bath (as the combined chromium plating bath) containing a trivalent chromium compound and a hexavalent chromium compound such that the total amount of trivalent chromium ions and hexavalent chromium ions is 60 to 140 g/L and the amount of hexavalent chromium ions is 5 to 40 g/L and the amount of hexavalent chromium ions accounts for 5 to 35 wt % of the total amount of chromium ions, and also containing organic carboxylate ions in an amount of 50 to 400 g/L, and preferably containing sulfate ions in an amount of 20 to 200 g/L, and having a pH value of 1.8 to 2.6.

On the other hand, the conventional combined chromium plating bath employs an insoluble anode of lead, lead alloy, carbon, titanium, platinum-coated titanium, or the like. Such anode generates oxygen on its surface and the resulting oxygen readily oxidizes trivalent chromium ions (Cr³⁺) into hexavalent chromium ions (Cr⁶⁺), which causes the concentration of hexavalent chromium ions in the plating bath to increase beyond the control limit in a relatively short period of time, thereby resulting in defective plating. Specifically, a lead anode suffers a disadvantage that trivalent chromium ions are oxidized into hexavalent chromium ions, which have to be reduced so as to restore the initial concentration. This results in troublesome bath maintenance and defective plating due to lead ions or tin ions dissolved in the plating bath, and also lead slime may be generated which is unfavorable from the environmental point of view. A carbon anode also suffers the disadvantage that trivalent chromium ions are oxidized into hexavalent chromium ions, which makes it necessary to restore the initial level. This leads to troublesome bath maintenance. In addition, upon oxidation and corrosion, the carbon anode releases fine particles which float in the plating bath. Those fine particles would stack to the object being plated, so the fine particles have to be filtered out. These problems are disadvantageous to plating control. As for a platinum-coated titanium anode, it also suffers the disadvantage of trivalent chromium ions being oxidized into hexavalent chromium ions, which leads to the necessity of restoring the initial level. This results in troublesome bath maintenance. In addition, there may be a loss of expensive platinum due to corrosion.

Consequently, the present inventors studied and found that use of an anode at least having on its surface an iridium oxide-containing coating suppresses oxidization of trivalent chromium ions into hexavalent chromium ions although it generates oxygen as in the foregoing cases.

Incidentally, there has been known the anode with an iridium oxide-containing coating film, and there has been proposed its use for the plating bath containing chromic acid or the plating bath containing a trivalent chromium compound. (See Patent Document 5, JP-A H03-260097 for the former, and see Patent Documents 6 and 7, Japanese Patents Nos. 3188361 and 3810043 for the latter.) However, no attempt has been made to use the foregoing anode with an iridium oxide-containing coating film for the combined plating bath.

It was found that the anode with an iridium oxide-containing coating film prevents the oxidization of trivalent chromium ions into hexavalent chromium ions when it is used with the combined plating bath. However, this effect diminished with time as the electrolysis continued, with an unexpected increase in the amount of hexavalent chromium ions beyond the above-mentioned control limit.

The present inventors further studied the phenomenon and revealed that the increased concentration of hexavalent chromium ions was caused by lead ions in the plating bath rather than by the anode itself.

The plating bath contains the lead ions that have entered the plating bath from the raw materials of the plating bath. Presumably, when such lead ions increase in concentration beyond the level of 2 mg/L by supplying materials to the plating bath, they are oxidized to lead oxides at the anode, and this lead oxides attach to the anode and functions as an electrode catalyst that brings about the electrooxidation of trivalent chromium ions into hexavalent chromium ions.

Therefore, the foregoing is a probable reason why the iridium oxide did not work as expected. The present inventors continued studies, and revealed that lead ions produce substantially no adverse effect when their concentration in the plating bath is no higher than 2 mg/L, so that the lower concentration of lead ions permit the concentration of hexavalent chromium ions to be kept at the above-mentioned optimal level required for stable chromium plating for a long period of time.

Accordingly, the present invention provides the chromium plating methods described below.

[1] A chromium plating method comprising the steps of:

immersing an object to be plated into an acidic chromium electroplating bath comprising:

a trivalent chromium compound and a hexavalent chromium compound such that the total amount of trivalent chromium ions and hexavalent chromium ions is 60 to 140 g/L, the amount of hexavalent chromium ions is 5 to 40 g/L, and the amount of hexavalent chromium ions accounts for 5 to 35 wt % of the total amount of chromium ions; and

organic carboxylate ions in an amount of 50 to 400 g/L; and

a concentration of lead ions in said acidic chromium electroplating bath being no greater than 2 mg/L,

performing electrolysis by using an anode having an iridium oxide-containing coating at least on the surface thereof.

[2] The chromium plating method of [1], wherein the trivalent chromium compound is chromium organic carboxylate or a mixture of chromium sulfate and chromium organic carboxylate complex, said mixture containing said chromium organic carboxylate complex in an amount that accounts for, in terms of trivalent chromium, 50 wt % or more of the total amount of trivalent chromium. [3] The chromium plating method of [1] or [2], wherein the chromium plating bath further comprises sulfate ions in an amount of 20 to 200 g/L and has a pH of 1.8 to 2.6. [4] The chromium plating method of any one of [1] to [3], wherein the chromium plating bath is halogen free. [5] The chromium plating method of any one of [1] to [4], wherein plating is accomplished in such a way that the anode and the object to be plated are immersed in the same plating bath of a plating tank without being separated from each other by a diaphragm.

Advantageous Effect of the Invention

The present invention makes it possible to obtain a good chromium plated film in a stable manner over a long period of time, with very easy maintenance of plating bath.

EMBODIMENT FOR CARRYING OUT THE INVENTION

A chromium plating method according to the present invention employs an acidic combined plating bath which contains a trivalent chromium compound and a hexavalent chromium compound as the chromium source and also contains carboxylate ions and further optionally contains sulfate ions as a stabilizer or a conductive salt.

The trivalent chromium compound should preferably be a complex of chromium with organic carboxylic acid. Examples of the organic carboxylic acid include oxalic acid, citric acid, formic acid, acetic acid, malonic acid, succinic acid, and lactic acid. Preferable among them are oxalic acid, citric acid, formic acid, and acetic acid. A complex of chromium with oxalic acid is particularly desirable. Incidentally, a preferable example of the complex of trivalent chromium with organic carboxylic acid is one obtained by mixing chromic acid (CrO₃) and the above-mentioned organic carboxylic acid in their aqueous solution and reducing the chromic acid with the organic carboxylic acid so that the resulting complex is free of hexavalent chromium. (See Japanese Patent Application No. 2008-294007.)

Other examples of the trivalent chromium compound include inorganic salts of trivalent chromium, particularly chromium sulfate. In the case where an inorganic chromium salt (such as chromium sulfate) is the only source of chromium, hydrogen occurs during plating due to electrolysis of water. This hydrogen makes the interface of the cathode strongly alkaline and hydrolyzes chromium sulfate, thereby forming chromium hydroxide and basic chromium sulfate. This prevents chromium plating of practical use.

On the other hand, the organic carboxylic acid forms a complex with trivalent chromium ions, thereby preventing and buffering hydrolysis of trivalent chromium ions, and it also functions as a pH buffer for the plating bath. Therefore, if an inorganic chromium salt (such as chromium sulfate) is to be used, it is preferably used in combination with a complex of chromium with organic carboxylic acid.

The total amount of trivalent chromium ions in the plating bath is preferably 55 to 135 g/L, particularly 72 to 112 g/L. The chromium complex of organic carboxylic acid is preferably used in such an amount that the ratio (by weight) of the trivalent chromium (as metal) in the complex to the total trivalent chromium (as metal) is from 0.5 to 1, particularly from 0.6 to 1, with the remainder being the inorganic chromium salt. It is desirable to use in combination a chromium complex of organic carboxylic acid and chromium sulfate, because a new initial plating bath forms a plated film having a thickness larger by 20% than that obtained in the case where the chromium complex of organic carboxylic acid is used alone. In the case of combination use, they are preferably used in such an amount that the ratio of trivalent chromium in the complex to trivalent chromium in chromium sulfate (trivalent chromium in the complex: trivalent chromium in chromium sulfate) is from 5:5 to 10:0, particularly from 6:4 to 10:0 (by weight).

On the other hand, the hexavalent chromium compound should preferably be chromic acid (CrO₃) or dichromic acid or a salt thereof. The amount of the hexavalent chromium compound should be 5 to 40 g/L, preferably 7 to 20 g/L (in terms of hexavalent chromium ions). The amount in this range is adequate for satisfactory chromium plated film. With an amount outside this range, the resulting plated film will be poor and irregular in appearance.

The total amount of the trivalent chromium ions and the hexavalent chromium ions should be 60 to 140 g/L, preferably 80 to 120 g/L. The amount in this range is adequate for satisfactory chromium plated film. With an amount outside this range, the resulting plated film will be poor and irregular in appearance.

In this case, the amount of hexavalent chromium ions should account for 5 to 35 wt %, preferably 10 to 25 wt %, of the total amount of chromium ions.

The amount in this range is adequate for satisfactory chromium plated film. With an amount outside this range, the resulting plated film will be poor and irregular in appearance.

According to the present invention, the chromium plating bath should contain organic carboxylate ions in an amount of 50 to 400 g/L, preferably 100 to 300 g/L. Examples of the organic carboxylate source includes from oxalic acid, citric acid, formic acid, malonic acid, succinic acid, lactic acid, and the like. Preferable among them are oxalic acid, citric acid, formic acid, and acetic acid. The organic carboxylate ions form a complex of organic carboxylic acid with the trivalent chromium ions mentioned above. With an amount less than 50 g/L, they do not form the chromium complex of organic carboxylic acid sufficiently. This leads to plated film having a poor and irregular appearance. By contrast, with an amount more than 400 g/L, they form excess complexes, thereby preventing trivalent chromium ions from being released easily. This leads to plated film having a poor and irregular appearance. Incidentally, when trivalent chromium ions are oxidized into hexavalent chromium ions by the anode in the plating bath, so that the concentration of hexavalent chromium exceeds the adequate range, it is possible to restore the adequate range by adding the above-mentioned carboxylic acid for reduction of hexavalent chromium ions.

According to the present invention, the chromium plating bath may be additionally incorporated with sulfate ions (for a stabilization or conductive salt) in an amount of 20 to 200 g/L, preferably 30 to 150 g/L. Examples of the sulfate ions source include sodium sulfate, potassium sulfate, ammonium sulfate, magnesium sulfate, and the like. Preferable among them are sodium sulfate and ammonium sulfate. Sulfate ions in an excessively low concentration may cause the plating voltage to unduly increase. Conversely, sulfate ions in an excessively high concentration may result in a slight decrease in thickness of the plated film.

According to the present invention, the chromium plating bath may be optionally incorporated with an anti-pit agent for removing bubbles from the surface of the plating film.

According to the present invention, the chromium plating bath preferably does not contain any halogens except for those as impurities. Thus, the chromium plating bath does not contain halides. The plating bath containing halides is impractical because an odor of halogen gas is emitted. Moreover, the halides may lead to some troubles such as deterioration of the appearance of the plated film; corrosion of the chromium plated film and the object to be plated, caused by a compound formed with halogen gas which is dissolved in the plating bath; corrosion of the object to be plated, caused by halogen ions; etc.

According to the present invention, the chromium plating bath should essentially be free of lead. The range of practically permissible content of lead ions is 2 mg/L and below. The lower the better. As mentioned above, the plating bath contains lead ions which originate from the raw materials of the plating bath and the contaminant entering from the outside. Lead ions in an amount exceeding 2 mg/L cause oxidation of lead ions on the anode and give lead oxide which attaches to the anode and functions as the electrode catalyst. As the result, trivalent chromic ions turn into hexavalent chromic ions through electrolytic oxidization, which is detrimental to the inherent performance of the iridium oxide-containing anode (to be mentioned later). This situation can be avoided by keeping the concentration of lead ions not higher than 2 mg/L and reducing lead ions through electrolysis or substitution reaction with metal. The iridium oxide-containing anode can then produce its inherent performance (reaction in 100% efficiency for oxygen generation).

In order to maintain the concentration of lead ions not higher than 2 mg/L, it is desirable to minimize the amount of lead ions originating from the raw materials for the plating bath by selecting high-purity raw materials. If the procedure is inapplicable, alternatively, lead ions may be removed by any known method such as treatment with ion-exchange resin or chelating resin, electrolysis, and substitution deposition (which involves immersion of metal such as iron, nickel, cobalt, and copper in the plating bath).

According to the present invention, the chromium plating bath should be acidic, preferably with the pH value being 1.8 to 2.6, particularly 2.0 to 2.3. A suitable pH adjuster to keep the desired pH value is ammonia or hydroxide (such as NaOH, KOH, and chromium hydroxide) for pH increase and sulfuric acid for pH decrease.

According to the present invention, chromium plating in the chromium plating bath mentioned above may be accomplished in the usual way by performing electrolysis with a prescribed current density in the plating bath in which are immersed the object to be plated (which functions as the cathode) and the anode. The anode used in the present invention has an iridium oxide-containing coating at least on its surface.

The anode is preferably composed of a substrate (conforming to the shape of the anode) and a coating film which is formed on the surface of the substrate. The substrate may consist of titanium, tantalum, zirconium, or niobium, or alloy thereof. The coating film may be a film consisting of iridium oxide alone, or a composite film consisting of iridium oxide in combination with an oxide of Ta, Si, Mo, Ti, Zr, or W, or with any other oxide to improve the corrosion resistance of iridium oxide. The coating film should not contain tin oxide or lead oxide which is intended for anodic oxidization of trivalent chromium in a plating bath containing hexavalent chromium. In the case of the composite film, the content of iridium oxide should be 20 to 95 wt %, particularly 30 to 90 wt %, so that the iridium oxide sufficiently performs. The iridium oxide-containing coating film (alone or in combination with another oxide) should be applied in an amount of preferably 0.2 to 1 g/de, particularly 0.2 to 0.6 g/dm².

Using the iridium oxide-containing anode, the reactions occurring at the anode can be rendered solely (almost 1000) oxygen generation, while preventing anodic oxidation and anodic reaction of the other components in the plating bath. This is due to the fact that the iridium oxide-containing anode has a low overvoltage for oxygen generation, which leads to a remarkable catalytic action for oxygen generation, anodic reaction involving almost entirely oxygen generation, very little oxidation of trivalent chromium ions into hexavalent chromium ions at the anode, and the limited possibility of the organic acid undergoing oxidative decomposition at the anode. Incidentally, lead anode, carbon anode, and platinum-coated anode bring about all of the oxygen generation, the oxidation of trivalent chromium, and the oxidative decomposition of organic acid. In the case of these anodes, the anodic oxidation of trivalent chromic ions is proportional to the amount of electrolysis, so that all of trivalent chromium ions turn into hexavalent chromium ions eventually.

The iridium oxide-containing anode mentioned above produces the effect of limiting the generation of hexavalent chromium ions, preventing the oxidative decomposition of organic acid, extending the life of the plating bath, facilitating the plating bath maintenance, and keeping the concentration of hexavalent chromium ions within an adequate range. In addition, the combined plating bath containing both hexavalent chromium ions and trivalent chromium ions permits the concentration of hexavalent chromium ions to extend over a broad range, which makes it easy to maintain the plating bath.

Chromium plating with the above-mentioned chromium plating bath and iridium oxide-containing anode is preferably accomplished under the conditions of plating temperature of 35 to 60° C., particularly 40 to 50° C., and cathode current density of 5 to 15 A/dm² particularly 6 to 12 A/dm². The chromium plating includes rack plating and barrel plating (with intermittent current application). An anode current density is preferably 3 to 20 A/dm² particularly 5 to 14 A/dm². The plating bath may be continuously filtered. This continuous filtration serves also as slow stirring to eliminate temperature variation. The duration of plating varies depending on the film thickness required. The film thickness may be increased by extending the plating time. Incidentally, the cathode current efficiency in the chromium plating is usually 5 to 20%.

The chromium plating method according to the present invention does not need any diaphragm such as ion-exchange resin membrane. This is desirable for practical plating because any diaphragm complicates plating operation and maintenance. The chromium plating method that employs the iridium oxide-containing anode prevents the formation of hexavalent chromium ions and the anodic decomposition of organic acid. This obviates the necessity of diaphragm and facilitates maintenance of the plating bath.

EXAMPLES

The present invention will be described in more detail with reference to Examples and Comparative Examples, which are not intended to restrict the scope of Examples.

Example 1

The following chromium plating bath was prepared.

<Composition of Chromium Plating Bath>

Chromium oxalate  78 g/L (in terms of Cr³⁺) Ammonium sulfate 120 g/L Chromic acid  20 g/L pH  2.2

The contents of trivalent chromium ions, hexavalent chromium ions, oxalate ions, and sulfate ions were as follows. Incidentally, the content of Pb ions was 1 mg/L.

Trivalent chromium ions  78 g/L Hexavalent chromium ions  10 g/L Oxalate ions 248 g/L (in terms of oxalic acid dihydrate) Sulfate ions  87 g/L

A composite iridium oxide anode consisting of a titanium plat which was coated with a mixture of iridium oxide and 30 mol % (in terms of metal) of tantalum oxide was used as an anode. An amount of the mixture including iridium oxide in the coating film is 0.5 g/dm² (in term of iridium metal). A plastic object processed up to a nickel electroplating is used as an object to be plated (cathode). Chromium plating was carried out for 10 minutes under the conditions of 10 A/dm² of cathode current density and 6 A/dm² of anode current density. The plating bath was circulated and filtered by a filtering device equipped with a polypropylene filter.

The plating gave a chromium plated film which has a good appearance and excellent corrosion resistance. Incidentally, the average thickness of the plated film was 0.5 m.

The anode was tested for anode current efficiency by performing electrolysis up to 100 AH/L. The results are shown in Table 1. As the result of electrolysis up to 100 AH/L, the concentration of hexavalent chromium ions increased. The current efficiency for hexavalent chromium ions was 7%, and the current efficiency for the decomposition of oxalic acid was 1%. The current efficiency for the generation of oxygen was estimated at 92% based on the assumption that it is the remainder of the foregoing two values.

Comparative Example 1

The same procedure as in Example 1 was repeated to perform chromium plating except that the composite iridium oxide anode was replaced by a lead anode. The resulting chromium plated film had as good appearance as that of Example 1.

The anode performance was evaluated in the same way as in Example 1. The results are shown in Table 1. It is noted that the efficiency for the generation of hexavalent chromium ions was 40%, the efficiency for the decomposition of oxalic acid was 10%, and the efficiency for the generation of oxygen was 50%. As compared with Example 1, Comparative Example 1 results in a higher efficiency for the generation of hexavalent chromium ions and a higher efficiency for the decomposition of oxalic acid. Thus, chromium plating in this manner needs a large amount of oxalic acid to reduce the concentration of hexavalent chromium ions. This would lead to the frequent and troublesome maintenance of the plating bath.

Incidentally, the values of the anode current efficiency remained almost unchanged even when the lead anode was replaced by a Pt—Ti anode or carbon anode.

Example 2

The same procedure as in Example 1 was repeated to perform chromium plating except that the concentration of hexavalent chromium ions was changed to 20 g/L and the concentration of lead ions was changed to 2 mg/L. The resulting chromium plated film had as good appearance as that of Example 1.

Example 3

The same procedure as in Example 1 was repeated to perform chromium plating except that the composite anode was replaced by an anode made of iridium oxide alone. The resulting chromium plated film had as good appearance as that of Example 1.

Example 4

The same procedure as in Example 1 was repeated to perform chromium plating except that the chromium oxalate was replaced by chromium citrate. The resulting chromium plated film had as good appearance as that of Example 1.

Example 5

The same procedure as in Example 1 was repeated to perform chromium plating except that the plating bath was additionally incorporated with chromium sulfate in an amount of 5 g/L (in terms of Cr³⁺). The resulting chromium plated film had as good appearance as that of Example 1. In addition, the resulting plated film had an average thickness 1.2 times as large as that of Example 1.

Comparative Example 2

The same procedure as in Example 1 was repeated to perform chromium plating except that the content of Pb ions was changed to 10 mg/L. The resulting plated film was poor in appearance possibly on account of Pb ions.

Comparative Example 3

The same procedure as in Example 1 was repeated to perform chromium plating except that the content of hexavalent chromium ions was changed to 2 g/L. The result of plating was defective on account of the reduced concentration of hexavalent chromium ions which was lower than the lower control limit.

Comparative Example 4

The same procedure as in Example 1 was repeated to perform chromium plating except that the content of hexavalent chromium ions was changed to 50 g/L. The result of plating was defective on account of the increased concentration of hexavalent chromium ions which was higher than the upper control limit.

The test for evaluating anode performance carried out on Example 1 was also performed on Examples 2 to 5 and Comparative Examples 2 to 4. The results are shown in Table 1.

TABLE 1 Comparison of Anode Current Efficiencies Current Current Current efficiency efficiency efficiency for hexavalent for for chromium organic acid oxygen generation decomposition generation Remarks Example 1 7% 1% 92% Within control limits: Cr(VI) = 10 g/L, Pb = 1 mg/L Composite iridium oxide anode Example 2 7% 1% 92% Within control limits: Cr(VI) = 20 g/L, Pb = 2 mg/L Composite iridium oxide anode Example 3 7% 1% 92% Within control limits: Cr(VI) = 10 g/L, Pb = 1 mg/L Simple iridium oxide anode Example 4 7% 1% 92% Within control limits: Cr(VI) = 10 g/L, Pb = 1 mg/L Composite iridium oxide anode Example 5 7% 1% 92% Within control limits: Cr(VI) = 10 g/L, Pb = 1 mg/L Composite iridium oxide anode Comparative 40%  10%  50% Lead anode Example 1 Comparative 32%  8% 60% Within control limits: Cr(VI) = 20 g/L Example 2 Outside control limits: Pb = 10 mg/L Composite iridium oxide anode Comparative 7% 1% 92% Outside control limits: Cr(VI) = 2 g/L Example 3 Within control limits: Pb = 1 mg/L Composite iridium oxide anode, plating with poor appearance Comparative 7% 1% 92% Outside control limits: Cr(VI) = 50 g/L Example 4 Within control limits: Pb = 1 mg/L Composite iridium oxide anode, plating with poor appearance Remarks: The equivalent of chromium deposition in the three-electron reaction is 17.3. In other words, 17.3 g of chromium deposits with 1F = 26.8 AH. It follows, therefore, that 1.21 g of chromium deposits at a current efficiency of 7%, which translates into deposition of 0.045 g of chromium per AH [(1.21 g)/(26.8 AH)]. Consequently, the concentration of hexavalent chromium ions increases by 4.5 g/L in electrolysis with 100 AH/L.

A composition change in the plating bath and an appearance of the film were evaluated after the chromium plating was performed for 200 hours (100 AH/L electrolysis). The chromium plating was performed for 200 hours (100 AH/L electrolysis) by using the plating baths of Examples 1 to 3 and Comparative Examples 1 to 4, respectively. The results of the composition change in the plating bath and the appearance of the film are shown in Table 2.

TABLE 2 Composition change in plating bath and appearance of film after chromium plating for 200 hours Concentration Concentration of hexavalent of Appearance chromium organic acid of film Results Example 1 14.5 g/L 279 g/L Good Generation of hexavalent chromium and decomposition of organic acid were limited to a low level. Plating bath composition remained in an adequate range. Example 2 24.5 g/L 280 g/L Good Generation of hexavalent chromium and decomposition of organic acid were limited to a low level. Plating bath composition remained in an adequate range. Example 3 14.5 g/L 279 g/L Good Generation of hexavalent chromium and decomposition of organic acid were limited to a low level. Plating bath composition remained in an adequate range. Comparative 45.7 g/L 220 g/L Poor Concentration of hexavalent chromium Example 1 ions exceeded an adequate range. Film was poor in appearance. Comparative 40.6 g/L 230 g/L Poor Concentration of hexavalent chromium Example 2 ions exceeded an adequate range. Film was poor in appearance. Comparative  6.5 g/L 279 g/L Good Concentration of hexavalent chromium Example 3 ions increased to an adequate range. Film was good in appearance. Comparative 54.5 g/L 279 g/L Poor Concentration of hexavalent chromium Example 4 ions was originally beyond an adequate range. Film was poor in appearance. Remarks: Electrolysis for 200 hours at 0.5 A/L is equivalent to electrolysis with 100 AH/L. Regarding Comparative Example 3, since good plating is achieved when the concentration of hexavalent chromium is 5 g/L or greater, the appearance of the plated film was poor in the initial stage with a concentration of 2 g/L but it improved as the concentration increased to 6.5 g/L in the later stage of plating. 

1. A chromium plating method comprising the steps of: immersing an object to be plated into an acidic chromium electroplating bath comprising: a trivalent chromium compound and a hexavalent chromium compound such that the total amount of trivalent chromium ions and hexavalent chromium ions is 60 to 140 g/L, the amount of hexavalent chromium ions is 5 to 40 g/L, and the amount of hexavalent chromium ions accounts for 5 to 35 wt % of the total amount of chromium ions; and organic carboxylate ions in an amount of 50 to 400 g/L; and a concentration of lead ions in said acidic chromium electroplating bath being no greater than 2 mg/L, performing electrolysis by using an anode having an iridium oxide-containing coating at least on the surface thereof.
 2. The chromium plating method of claim 1, wherein the trivalent chromium compound is chromium organic carboxylate complex or a mixture of chromium sulfate and chromium organic carboxylate complex, said mixture containing said chromium organic carboxylate complex in an amount that accounts for, in terms of trivalent chromium, 50 wt % or more of the total amount of trivalent chromium.
 3. The chromium plating method of claim 1, wherein the chromium plating bath further comprises sulfate ions in an amount of 20 to 200 g/L and has a pH of 1.8 to 2.6.
 4. The chromium plating method of claim 1, wherein the chromium plating bath is halogen free.
 5. The chromium plating method of claim 1, wherein plating is accomplished in such a way that the anode and the object to be plated are immersed in the same plating bath of a plating tank without being separated from each other by a diaphragm. 